Oops, now that I think about it, there’s no need for triplication, which only complicates things. Duplicating every chromosome and dividing in 2 is the way to go, unless we also imagine a genetic material that is 3 stranded instead of 2 like DNA.
ETA: the key would still be in meiosis, where the second division goes from 2 3n cells to 6 1n cells.
Conversation on 3rd Rock after Sally discovers homosexuals:
Sally: You know how we said that having only two sexes must be boring?
Dick: Yeah.
Sally: I think some of them have found a loophole.
How about a 2 strand DNA, where AA, BB and AB are all different and viable genders. There could be a set up where AB are female and both AA and BB are male (or even flip flop that). So AA and AB could reproduct, as could BB and AB. But AA and BB couldn’t directly have offspring.
Besides the SF examples linked to above, there are at least two examples I know of where multiple genders (more than 3) have to hook up for sex. it was treated as a comedic look at human sexual politics in a story in Playboy circa 15-20 years ago, and as a cartoon strip (that ran for several pages) in National Lampoon.
I don’t know of any examples that really try to treat the mechanics of such science fictiion three-way sex, except where the third partner is a sort of catalyst or facilitator (as in Alien Nation, or in John Varley’s Titan series, during the time when Rocky has to catalyze the centaur eggs before they’ll impregnate. Otherwise, nobody wants to try and figure anything out. In asmov’s the Gods Themselves the beings are sort of ill-defined and fuzzy. The most cases, they simply describe the participants as crowding together in a multisexual orgey.
I think people are getting DNA and chromosomes mixed up.
You don’t get half DNA strands from each parent, you get whole chromosomes from each parent. A chromosome is a clump of DNA. You have IIRC 46 chromosomes. 23 of which are redundent. They have DNA for the same things as the other 23. You get 23 from each parent, and when you have kids your kids will each get half your DNA because they will get copies of half your chromsomes. Some might get the chromosome for blue eyes, and some might get the chromosome that has black eyes, but of the two chromosomes with eye color genes they will get one.
IANA biologist so someone correct me if I’m wrong.
In DNA an A segment on one side of the helix always connects to a T on the other. G connects to C etc. When the DNA strand is copied it’s split down the center, and this pairing is used to rebuild the missing side on each strand. This has about as much to do with sexual reproduction as it does with any tissue growth.
The trouble with a lot of these setups is that they don’t make any evolutionary sense. If you need a second type of male to “catalyze” reproduction, what’s in it for the catalyst male? And how did this setup evolve? In the case of “Alien Nation” the explanation is that the newcomers have been genetically engineered, and the third sex could be an artificially created method of limiting reproduction.
The other common setup, where you have a male who provides sperm and a female who provides eggs and a third gender that gestates the offspring suffers from the problem of what’s in it for the gestator. Why would an organism go to the trouble of gestating a baby that it isn’t the parent of? In the puppeteer example the puppeteers parasitize the third “gender”, so it isn’t an example of a third sex any more than a caterpillar infested with wasp larvae is a third gender of wasp.
For all the organisms that we know two genders is probably too many. Rather than two genders most species would be better off with one and a half. Sex is advantageous, but you only need a little bit to get the advantages of sexual reproduction. And so we see countless organsims that have both sexual and asexual methods or reproduction, but none that have three sexes.
I don’t see this as much stranger than say, worker bees not having the ability to reproduce. I agree that wasps laying eggs in caterpillars is not an example of a third gender, but what if the wasp lays it’s egg in another member of its species that has specialized to become a gestation machine? You have your male and female wasp, and a third gender that looks like a large caterpillar which munches on leaves and builds up a large reserve of nutrients which is used to feed the hatching larvae. Seems like a plausible setup to me.
But what does the third gender get out of the setup? I suppose it could work with some sort of haplodiploidy like sterile haploid hymenopterans. But it seems to me a mistake to call the gestator a third sex. It’s an infertile neuter, not a third sex.
Human females are becoming “gestational surrogates,” using their wombs to give birth to a couple’s child, or even a child for a gay couple using donor sperm and/or eggs. There are medical, financial, altruistic and psychological reasons. Some women like being pregnant,but don’t want another child and don’t want to give up a genetic child.
That’s why I postulate a third gender would be viable in a species with 3 sets chromosomes, or structures different then nuclear DNA/chromosomes.
Although what about a species with alot more emphases on mitochondrial DNA? Say instead of it being 5% it’s 40% or 30%. The first two genders pass on nuclear DNA, the third gets to pass on it’s mitochondria.
I think that’s debatable, depending on the exact mechanisms. I can imagine instances where the gestator would affect the offspring, either through environmental conditions or contribution of some of its cells and/or genetic material. I think that should qualify.
That’s an interesting idea too. Just goes to show even dealing with familiar biochemistry, there’s a lot of options. Not necessarily practical or even likely, but it’s not like evolution only results in practical features 
But in that case it is no longer a gestator any more than a female mammal is a gestator. And at that point we are back to the original question: how could this evolve? Why would a gestator evolve to contribute some (let’s say 10%) of its genetic materials when it could remain conservative and contribute half of the genetic material? What’s in it for the gestator that outweighs losing 40% of its reproductive potential? For such a setup to evolve a proto-gestator would have to be able to produce at least 41% more offspring than standard sexual reproductives.
But we are still left with the question: how could this possibly evolve? Why would any individual forego the ability to contribute 100% of nuclear and 100% of mitochondrial DNA in order to just contribute mitochondrial? And here we have another level of complexity:how could the mitochondrial donor possibly evolve even if it somehow exists? It has no ability to control the phenotype of its own offspring since that is controlled entirely by the nuclear DNA. It;s an evolutionary dead end and will rapidly be outcompeted by organisms that simply reproduce the old fashioned way.
For a third sex to evolve it would need to offer at least a 18% reproductive advantage to all the individuals involved since all individuals will lose 17% of their reproductive capacity by entering into the arangement. IOW if a standard coupling produces two offspring a triple sex partnership would need too produce four to be evolutionarily viable (technically it only need to produce 3.1, but births are quantum events so four becomes the minimum).
If the third sex doesn’t produce such an advantage then it is either a parasite or is being parisitised. The situation is then no different from the numerous viruses that contribute small amounts of genetic material to all organisms. And nobody calls a virus a third sex.
It’s really hard to imagine a scenario in which the third sex could possibly produce such a massive evolutionary advantage.
Well males pass on absolutely no mitochondria. Just ain’t room in the sperm for it.
A species with 40% of it’s DNA in it’s mitochondria would either be chock full of junk DNA or performing alot of otherwise nucleic functions in it’s mitochondria.
These functions would be time tested since more rapidly changing things would be better suited to chromosomal DNA which can be mixed and matched, and would also possibly indicate an environment where rapid reaction was crucial. Simply put it’d be the many small hands of the mitochondria floating near the cell membrane able to react sooner and in multiple ways vs a big central nucleous. A larger critter would be able to maintain some kind of homeostasis and shield it’s internals from the environment, but microscopic animals couldn’t.
Also it wouldn’t necessarily be one gender. Each organism could do all three gender rolls.
Plus a selfish gene setup might play out when an organism will develop into the 3rd gender to help it’s relatives reproduce, and indirectly pass on it’s own genes.
Firstly sperm are packed with mitochondria. How else did you think they move? It is the ovum that has very low mitochondrial density because it is entirely passive
Secondly sperm do pass on mitochondria, but only under conditions in which the ovum is mitochondrially derelict, either no mitochondria, defective mitochondria or mitochondria that can’t reproduce. We still aren’t sure how common paternal mitochondrial inheritance is but it seems to be a standard fall back defence against defective maternal mitochondria.
Which highlights the problem of a mitochondrial gender: mitochondrial inheritance is so worthless form an evolutionary POV that males only pass on that genetic material when there is no other choice. In fact it is now believed that this is because paternal mitochondria have a higher chance of being incompatible with the gemetes and may even trigger autoimmune responses.
Precisely, yet it would need ~40% of its phenotypic genetics in the genetic material committed by the third sex for the third sex to have any chance of evolving
Is that based on anything? I can;t see any logical reason for such a claim.
Quite simply cells are so small, the speeds of diffusion and active transport so relatively fast, the transmission trigger molecules across the organelle membrane so precise and the processes of transcription so exacting that a few extra microns couldn’t possibly make any difference to the rate of gene expression.
Exactly. A single celled organism always has all perts of the cell, whether nucleus or mitochondria, in close contact with the environment. The idea that the extra millisecond it takes to diffuse a hundred microns could have any effect on the rate of gene expression seems to contradict everything we know about the process.
I fail to see how this changes anything. You still need to decrease your genetic input into any offspring by 18%, it doesn’t matter whether you are a hermaphrodite or not. Why would any organism forfeit 17% of its reproductive fitness for no apparent advantage?
And the question that has been asked about 10 times in this thread is: How could this possibly happen? We all agree that it needs to happen for a third sex to evolve, but how could it possibly happen?As others have noted their are plenty of organisms form bees to birds to rats that have non-reproductive helpers in their colonies, but non ehave ever evolved athird sex. By definition such individuals must be non-reproductive. If they are successfully reproductive at all then they must rapidly evolve to become perfect reproductives simply because perfect reproductives will produce more offspring.
Have a look at the life cycle of a typical bumble bee colony to see what happens if helpers become reproductives.
Can you explain how any organism can be reproductive yet remain 17% below optimal reproductively? Isn’t such an organism going to rapidly lose out, either to any sibling that is 17% more efficient at reproducing or to those groups of organisms where helpers devote no energy to personal reproduction?
Well this way out of my league so I’ll leave it at that. I do agree that a third gender is very unlikely with earth biology as we know it for most things, save for the fungus mentioned up thread.
What about ferns? IANABiologist, but as I understand it, a mature fern produces spores, neither male nor female, nor eggs or sperm, and the spores germinate into a form with both male and female parts, which must then reproduce sexually in order to produce another mature fern.
So, I could see the third gender being the mature spore producing gender who gives birth to both males and females who then have to mate in order to produce another mature spore producer.
But this isn’t a third gender, it’s just an alternation of generations. All plants and most algae have that life cycle. Even a few animals alternate freely between sexual and asexual generations.
Egads, I completely forgot about the Titan trilogy.
[spoiler]But it was because the alien being who altered the centauroids’ reproductive system to require Rocky’s saliva to activate their fertilized ova - and the burden of the responsibility drove her fairly insane - and definitely addicted (don’t blame her, but it was a logical substitute for warfare as population control, and she’s the one who wanted an end to the warfare between the centauroids and the “angels”). But in book 3, they discovered that (Robin’s ?) “miraculous” baby also possessed the ability.
But talk about complicated reproductive strategies! That was more than complex enough, I’d think, to enthrall the OP![/spoiler]
So I’m extremely dubious about whether that really counts as three genders; three participants, more like, since Rocky was contributing enzymes, not genetic material. But, KRSOradio, I think you might really enjoy that series. There are aliens in it who have an incredibly complex reproductive process, which they can vary in a startling number of different ways, in terms of the number of needed participants. Now that was ingenuity, creating that system. It seems almost like something that might have been dreamed up on LSD.
I fixed the spelling of the Good Doctor’s name.
Ummm. It was only the one, facilitating, gender that could be described as “ill-defined”, I think, and only because it had to sort of merge partly with each of the two others, in order for sex to take place. It’s been a very long time since I read that, but I’m pretty sure that each of the other two genders were pretty well defined - it was just that their bodies were sort of ovoid. Wasn’t it (another book I can’t get to)?
It has always seemed to me that book was more of an experiment. Wasn’t it written after he married Janet, and finally had a happier home life? Or merely after he met her? I mean, he’d never really done emotions before, that I can recall. Prior to that, (excluding some of his positronic robots), the only human character he ever wrote that seemed to me to have a real personality was the protagonist in Pebble in the Sky (the guy who gets hit by the beam of radiation at the beginning of the book, the one who loved the Robert Browning poem).
a) I don’t understand why many posters are insisting on some sort of genetic contribution by the third individual to be considered a separate sex (or even evolve)? You could have two haploid producing sexes that need some sort of an incubator individual to produce offspring. The incubator does not need to pass on their genes to evolve – nor do they have to have any genetic markers for being an incubator sex. Plenty of species have sterile offspring that fulfill various important tasks – remember, as long as giving birth to some offspring of the third gender gives your genes no disadvantage, the mutation stays.
b) Life on earth just happens to tend to have genetic differences between the two sexes but I don’t see how that’s a pre-requisite for having sexual reproduction.
Imagine a species with two sexes but nothing equivalent to X/Y chromosomes – rather the mother’s (or incubator’s) body arbitrarily (or not) picks the sex by supplying different hormones during different stages of development. Organs develop differently, produce different sexual characteristics, and you still need a male and a female to reproduce, but there is nothing about either parent’s haploid that would affect which sex the child will be.
c) It’s not hard to imagine a species that requires three or four genetically contributing genders. Reproduction and life cycles don’t have to be efficient or logical to evolve. One can even imagine something seemingly bizarre like a haploid individual stage, where a larval individual develops from a haploid, then mates with an adult, undergoes metamorphosis to become another adult.
Because that isn’t in any way unusual, it’s as common as muck. Mosquitoes or tapeworms fit your example perfectly. That doesn’t make you the third mosquito sex. It simply makes you a host for a parasite.
But it doesn’t make it a sex, it makes it a host. You may wish to look up a dictionary. An oak is not and never has been considered the third sex of a moss no matter how closely entwined the commensal relationship may be, no matter how dependent the parasite is upon its host nor how benign the parasite may be.
You are arguing that any host is the third sex of any obligately parasitic or commensal organism upon it. Since this is GQ the time has come for you to provide a reference for this assertion. Does anyone in the entire world agree that an oak tree is the third sex of a moss?
You mean exactly like crocodiles, or turtles, or many fish, or any of a plethora of other animal and plant species?
There’s no need to imagine this situation, it is an astoundingly common solution in the world all around you.
Since oyu seem to be able to be able to imagine this can you please elaborate on your imaginings. Because I can;t imagine how this could possibly be achieved for the reasons outlined above.
What is the advantage for any pair of individuals in allowing a third individual to contribute genetic material to this union? It lowers the reproductive potential of both the original couple by 17%, how do they recoup that loss?
Yes, they do. By definition they have to be more efficient than the ancestral condition, otherwise they would never compete with that condition. And they need to be logical insofar as they have to m comply with the laws of mathematics and physics.
But how can any organism be more efficient by reducing its reproductive fitness by 17%? Why would this organism not be rapidly outcompeted by its ancestral type that is 17% more efficient? What’ sin it for the very first tri-sex individual that will allow it to compete against its own di-sex siblings?
What do you mean by “mates with an adult”?
With a diploid adult? But again, how would this be advantageous to either party? The haploid only gets to contribute 33% of the offspring’s genetic material, compared to 50% if it was diploid, so will rapidly be outcompeted by its own diploid siblings. The diploid adult only gets 33% increased genetic diversity material for its offspring as opposed to 50% if it mates with a diploid, so once it will rapidly evolve to select against this new lifestage, making it even less viable evolutionarily.
With a haploid adult? That is no different to the alternation of generations seen in plants. Indeed it is no different to the production of gametes seen in most animals: two haploid forms fuse to form a diploid. Nobody says that sperm and ova are third and fourth sexes. They are gametes are at best alternate generational forms.
I still can’t see any possible way in which this situation has any advantages over the ancestral condition.